The effectiveness of topspins, slices, and the like executed by expert tennis players in matches depends, to a large extent, on the amount of nap on the tennis ball. There is no problem with new tennis balls since a proper nap is initially provided. However, even after a few games of hard tennis with new tennis balls, some of the nap wears off on the surface of the tennis ball, thereby affecting its path through the air, its friction when bouncing from the court surface, and its aerodynamic characteristics when sliced or provided with a top or under spin.
Since the useful life of tennis balls generally exceeds the time for the nap to become ineffective, it has long been the practice to renap the ball by simply combing the surface with a bristle type brush. This operation is somewhat time consuming particularly if all surfaces of each tennis ball are to be properly renapped.